Chair base



FIG. 1 is a top perspective view of the chair base;

FIG. 2 is a front view of the chair base of FIG. 1;

FIG. 3 is rear view of the chair base of FIG. 1;

FIG. 4 is a right side view of the chair base of FIG. 1;

FIG. 5 is a left side of the chair base of FIG. 1; and,

FIG. 6 is a top plan view of the chair base of FIG. 1.

The portions shown in broken line are not part of the design as claimed. 

I claim the ornamental design for a chair base, as shown and described. 